The role of lung ultrasonography in the assessment of overhydration in maintenance hemodialysis patients

Abstract Purpose Existed methods like biochemical markers improve the accuracy of fluid evaluation for the maintenance hemodialysis patients, but none of them has become the gold standard. This study aimed to evaluate the potential of lung ultrasonography as a useful tool for monitoring the volume status of the patients. Methods A total of 88 patients undergoing maintenance hemodialytic were enrolled in this prospective observational study. Patients were divided into three groups: overhydration (OH), normohydration, and hypohydration according to bioimpedance spectroscopy. Lung ultrasonography parameters, echocardiography parameters, and clinical characteristics of three groups were analyzed. After an average follow-up of 433 days, all-cause mortality among groups was compared. Results The total number of lung comets was statistically reduced in patients after dialysis (Z= −6.891, p < 0.001). This reduction was related to ΔOH (OH – ΔW (the weight gain from dry weight)) and echocardiographic parameters, which proved the relationship among the comet-tail, hydration status of body and cardiac performance. The Kappa consistency test showed that lung ultrasonography and bioelectrical spectroscopy had moderate consistency. ROC analysis showed that the best cut-point of lung comet is 13. The pre-/post-dialysis lung comet-tail, cardiac function and total body impedance with all-cause mortality was investigated. Kaplan–Meier’s analysis revealed that the all-cause mortality was higher in lung congestion patients. Conclusions This study proposes a potentially reliable lung ultrasonography method for estimating fluids overload, which also has implication value of all-cause mortality.


Introduction
Dialysis adequacy contains the elimination of toxins and fluid, is one of the most important objectives in end-stage kidney disease (ESKD) patients. Although much progress has been made in toxin assessment, there are still significant limitations in liquid assessment. The hypohydration causes hypotension and the overhydration (OH) is directly associated with left ventricular hypertrophy, both of which can result in high mortality and morbidity of cardiovascular disease in ESKD patients [1][2][3]. Currently, the 'dry weight' is used to name the standard weight in patients with maintenance hemodialysis, which means the lowest weight they can tolerate without symptoms of OH and hypohydration. However, many nephrologists in most dialysis centers assess the 'dry weight' based on clinical symptoms and signs. This subjective clinical method is imprecise and does not consider nutritional status. When the setting is higher than the actual 'dry weight', maintenance hemodialysis patients are characterized by a condition of volume overload, thereby leading to refractory hypertension, left ventricular hypertrophy and congestion heart failure [4,5]. On the contrary, they may be characterized by a state of hypohydration, such as hypotension, muscle twitches, vomit, and so on [2].
Several techniques have been used in clinic and have shown great impact on fluid evaluation, including biochemical markers, blood volume monitoring, vena cava diameter, bioimpedance spectroscopy, etc. [1,6,7]. All these methods may improve the accuracy of fluid evaluation, but none of them has become the gold standard. Lung ultrasonography has recently been proved to be useful for defining extravascular lung water (ELW), which is a relatively small but basic component of total body fluid. ELW represents the fluid content of the lung interstitium and it is closely associated with the filling pressure of the left ventricle. Therefore, lung ultrasonography examination of interstitial imbibitions or pleural effusion, decompensated heart failure, and rapid fluid clearance in maintenance hemodialysis patients has received more and more attention in clinical research [8][9][10]. The aim of this study was to evaluate the functional role of lung ultrasonography in detecting total body fluid and predicting the all-cause mortality in ESKD patients. Thus, we performed the lung ultrasonography combined with bioimpedance spectroscopy and echocardiography in a group of maintenance hemodialysis patients of Shanghai, China, in a cohort from a single hemodialysis unit.

Participants
Eighty-eight patients who received maintenance hemodialytic treatment were recruited at the Dialysis Unit of Shanghai Changzheng Hospital from 1 July 2014 to 30 June 2015. Hemodialysis therapy was performed using Fresenius FX 80 dialyzer (1.5 m 2 ; Fresenius Medical Care, Bad Homburg, Germany) for 4 h, three times a week. Biochemical parameters were determined on the first Monday or Tuesday of each month, per-dialysis, after the long interval of dialysis. All participants were informed and signed the consent. All protocols were carried out following the principles of the Declaration of Helsinki, and authorized by the Ethics Committee of Shanghai Changzheng Hospital (20211226).

Inclusion and exclusion criteria
The inclusion criteria: (1) the patients who had received long-term hemodialysis treatment for at least 3 months; (2) over 18 years old; (3) the patients who had received standard bicarbonate dialysis three times a week. The exclusion criteria: (1) pulmonary disease: such as interstitial lung disease, severe pneumonia, pulmonary tumor, and acute severe pulmonary edema; (2) cardiac disease: organic cardiopathy/severe congestive heart failure; (3) metallic joint prostheses, cardiac pacemakers or stents; (4) limb amputations or limb skin defect; (5) other electrical examinations were in progress, such as ambulatory blood pressure monitoring.

Procedures and evaluations
The lung ultrasonography examination was started 30 min before and after dialysis. The examination was performed utilizing the echographic equipment with 3.0 MHz convex probe, and the patients were in the supine or semisupine positions. The main scanning area is the second to fifth intercostal space on the right side, the side of the breastbone to the mid-axillary line, and the second to fourth intercostal space on the left side. The comet-tail sign was defined as the wedge-shaped echogenic artifact, and from the transducer to the limit of the screen (Figure 1). At each scan site, the signs of comet-tail were calculated, and the sum of these signs was a score, which represents the degree of extravascular fluid in the lung.
In the above 28 regions, the comet-tail signs were counted and the sum of the signs was a score, which denoting the degree of extravascular fluid in the lung.
The total body water (TBW) volume was estimated by bioimpedance spectroscopy apparatus (body composition monitor; Fresenius Medical Care, Bad Homburg, Germany) 30 min before dialysis and 30 min after dialysis. The electrodes were connected to the non-fistula forearm and ipsilateral ankle of patients, and the patients were in supine or semi-supine positions. The multi-frequency bioimpedance spectroscopy can distinguish the resistance among the TBW, the extracellular water (ECW), and the intracellular water (ICW). Then, TBW, ECW, ICW, and OH were determined through mathematical modeling. To facilitate the comparison among the patients, the OH that normalized to interdialytic weight gain (DOH ¼ OH -DW) was determined. Then patients were divided into three groups: OH (DOH > 1), normohydration (-1 DOH 1) and hypohydration (DOH< À1).
Echocardiographic measurement was performed 30 min before dialysis treatment, the following indicators were measured: ejection fractions, fractional shortening, fractional shortening, left atrial volume, and left ventricular end-diastolic volume.

Statistical analysis
Data were presented as mean ± standard deviation (SD), or median and interquartile range. Normally distributed data were analyzed by t-test, paired t-test or ANOVA test, and non-normally distributed data were analyzed by Wilcoxon's signed rank test. Significance for associations was analyzed by Pearson's correlation or Spearman's correlation. The agreement between categorical variables was measured by kappa statistics. The receiver operating characteristic curve (ROC) analysis was performed to evaluate the feasibility of the lung ultrasonography in assessing body water volume. The Cox regression analysis and Kaplan-Meier's analysis were used to investigate the prognostic value of the lung ultrasound in predicting mortality. p < 0.05 was considered as statistically significant.

Pre-dialysis
The patients were divided into three groups according to DOH: OH (DOH > 1), normohydration (-1 DOH 1) and hypohydration (DOH< À1). The demographic and clinical characteristics of the study population are summarized in Table 1. Compared with normohydration and hypohydration patients, there were more OH patients with 24-h urine volume (625 mL; p < 0.001) and pedal edema (p ¼ 0.004). The mean weight gain from dry weight of OH patients was less (p ¼ 0.004) and dialysis vintage was shorter (p ¼ 0.011). The albumin and hemoglobin content of OH patients was significantly lower than that of normohydration and hypohydration patients (p < 0.001, p ¼ 0.003). There were significantly more lung comets in OH patients than that in normohydration and hypohydration patients (p < 0.001). However, there was no significant difference in lung comets between normohydration patients and hypohydration patients.

The consistency between lung ultrasonography and bioimpedance spectroscopy
The Kappa consistency test was utilized to evaluate the consistency between lung ultrasonography and bioimpedance spectroscopy in all patients and non-peripheral edema patients. These patients were classified into two parts according to the comet tail sign: absent or mild pulmonary congestion (comet tail sign <14) and severe pulmonary congestion (comet tail sign !14). Furthermore, these patients were also classified into two parts according to bioimpedance spectroscopy apparatus: normohydration (jDOHj 1) and non-normohydration (jDOHj>1). In all patients group, Kappa consistency test showed that lung ultrasonography and bioelectrical spectroscopy had moderate consistency (Kappa ¼ 0.621, p < 0.001), while the patients without obvious peripheral edema showed better consistency (Kappa ¼ 0.706, p < 0.001). According to the ROC analysis, the comet tail sign may be an indicator for distinguishing clinically significant OH patients from non-hyperhydration patients, and identifying the degree of mild and severe pulmonary congestion. The ROC analysis revealed high accuracy (area under roc curve (AUC): 0.841 (0.758-0.925)) in detecting fluids overload (Figure 2). When the cut-point of lung comet was 13, the sensitivity (0.660) and specificity (0.950) of lung ultrasonography for detecting the fluid overload were the best.

Survival
During the follow-up period, 10 patients died and four patients were lost to follow-up. The median of observation time was 433 (346, 538) days. Kaplan-Meier's analysis showed that patients with severe lung congestion had a higher mortality than patients with absent or mild lung congestion (p ¼ 0.048) (Figure 3). Furthermore, the Cox regression analysis was used to investigate the risk factors of death. The Cox regression showed that the comet-tail score was a risk factor for death in dialysis patients (OR ¼ 3.909, p ¼ 0.048). Then, we used a multivariate Cox model, which included the comet-tail score, NYHA class, hydration status, basal epidemiological (age, sex, BMI, edema, diabetes, vascular access, and so on) and some lab data (hemoglobin, albumin, and phosphorus). The results showed that age (OR ¼ 1.157, p ¼ 0.010) maintained a significant correlation with survival time (Table 3). While comet-tail score (OR ¼ 7.704, p ¼ 0.076) and hydration status (OR ¼ 5.339, p ¼ 0.390) had no statistical significance.

Discussion
The lung ultrasonography has been considered as a highly sensitive method for detecting pulmonary edema in dyspnea patients [11,12]. The interlobular septum thickens under lung congestion, and then the ultrasound beam is reflected at the septum. The reflection of the beam creates the phenomenon of lung comets [13]. Due to volume overload, dialysis patients often experience lung imbibitions similar to pulmonary edema. Therefore, we believe that this principle can also apply to maintenance hemodialysis patients. The lung ultrasonography may be a potentially useful tool for monitoring the volume status of maintenance hemodialysis patients, which have been roughly assessed by traditional methods in the past.
The main findings of this study were: (1) comet tails was significantly reduced after hemodialysis. This reduction was significantly related to DOH and echocardiographic parameters, demonstrating the direct relationship among the comet-tail, hydration status of body and cardiac performance. Mallamaci et al. have also reported that the rapid reduction of the lung comet is only closely related to the altered left ventricle performance, but scarcely associated with hydration status [14]. Donadio et al. have reported the opposite result, that is, the dynamic change of B-lines is correlated with extracellular, but has nothing to do with intra-cellular water index [15]. However, Siriopol et al. have demonstrated that the reduction is not correlated with any echocardiographic or bioimpedance parameters [10]. ELW is known as a relatively small but fundamental component of total body fluid, and is associated with the filling pressure of the left ventricle. Therefore, the lung comet depends on cardiac function and circulating volume. When the heart function is normal, lung comet is the best hemodynamic parameter reflecting the volume of circulation. (2) We compared the performance of lung ultrasound and bioimpedance to assess volume status to better clarify the relationship between the different methods. The Kappa consistency test showed that the lung ultrasonography and bioelectrical spectroscopy had moderate consistency. Considering that patients with peripheral edema were able to distinguish volume overload or not, we also performed a subgroup Kappa consistency test in patients without peripheral edema. Surprisingly, the subgroup showed better consistency. Furthermore, bioelectrical spectroscopy showed that the best cut-point for lung comets was 13. So far, a gold standard for dry weight assessment has not yet been established. Therefore, we have to choose the bioimpedance as a comparison method in this study, which has been used clinically for many years [1,[16][17][18]. It distinguishes TBW, ECW, and ICW by injecting electric current into the body, and finally defines the individual hydration status through certain mathematical modeling. As a consequence, it does not take heart function into consideration. That may be the main reason for moderate consistency between lung ultrasonography and bioelectrical spectroscopy. (3) A implication value of all-cause mortality. This work holistically investigated the pre-/post-dialysis lung comet-tail, cardiac function and total body impedance with all-cause mortality. Despite the negative reactions in the multivariate Cox model, lung congestion patients had a higher all-cause mortality, which opened up promising avenues for future trials. Dialysis patients had a greater risk of all-cause mortality as compared with patients without kidney disease. Siriopol et al. have enrolled 96 patients from a single hemodialysis unit and followed up for a median of 405.5 days [10]. The mortality of severe lung congestion (>30) is higher than that of the other groups, and the lung comet score before hemodialysis has a significant  [19]. In an international, multi-center randomized controlled trial, Zoccali et al. investigated a lung ultrasound-guided treatment could improve recurrent episodes of decompensated heart failure and cardiovascular events [20]. In our research, the Kaplan-Meier analysis showed that patients with severe lung congestion had a higher mortality than patients with absent or mild lung congestion. The Cox regression showed that the comet-tail score was a risk factor for death in dialysis patients (OR ¼ 3.909, p ¼ 0.048). However, the multivariate Cox model which included the comet-tail score, NYHA class, hydration status, basal epidemiological (age, sex, BMI, edema, diabetes, vascular access, and so on) and some lab data (hemoglobin, albumin, and phosphorus), showed that only age (OR ¼ 1.157, p ¼ 0.010) maintained a significant correlation with survival time. The main reasons of negative respond in the multivariate Cox model might be the most enrolled patients were absent or mild lung congestion, and the follow-up period was shorter.

Limitation
The limitations of our study were the small number of enrolled patients from a single dialysis center and the short follow-up period. Moreover, the lung ultrasonography only provided information about OH, not information about hypohydration. The lung comet in MHD patients was not fixed and it could be differed through enhancing ultrafiltration. It would be more reasonable to monitoring with serial measurements. Finally, we did not collect the non-fatal events and specify the cause of death. Therefore, more large-scale studies are needed to evaluate the value of lung ultrasonography. The cutoff for diagnosis of volume overload also needs to be ensured in following studies.